This invention relates to the art of molding articles of organic plastic material from a parison, particularly under conditions imparting multi-axial orientation, and has for its principal objects the improved adjustment of the temperature of the parison, facilitating the transfer of the parison from one mold to another, and the provision of improved, oriented hollow articles.
The art teaches various methods and apparatus for obtaining blow molded articles of organic plastic material from a pressure molded parison, such as in U.S. Pat. No. 3,349,155 and U.S. Pat. No. Re. 27,104. Generally these methods are characterized by forming a parison in a parison mold on a blow core, placing said formed parison and blow core into a blow mold and expanding said parison in the blow mold by means of fluid pressure.
While the blow molding operation tends to impart orientation to the article, the degree and type of such orientation is difficult to control and therefore it is difficult to obtain the advantageous properties in the article that multi-axial orientation is capable of providing if said orientation is carried out under the most desirable conditions of temperature distribution by stretching and circumferentially expanding said parison.
It is known that the control or orientation depends largely upon the control of the temperature of the parison just prior to orientation. It is found that such temperature control is best obtained by enclosing the parison prior to the orienting step in an environment, such as a heater or a mold whose surfaces may provide conductive or radiant heat transfer to the corresponding surfaces of the parison.
Plastic articles exhibiting the properties of multi-axial orientation, e.g., bottles and other hollow containers, are frequently made in two stages. The first of these two stages is the production of the parison usually by injection molding, or by other pressure molding procedures such as compression molding and extrusion. The first stage thus serves to convert the chosen plastic into a parison of predetermined shape, having a wall thickness distribution suitable for the production of the final article shape and usually also having a neck or rim configuration suitable to receive a closure appropriate for the said article. If injection molded, the parison is removed from the injection mold typically consisting of a mold cavity, a core and a neck (or rim) mold, by extracting it from the mold cavity, by extracting the core therefrom and, either before or after its transformation into the finished article, by releasing it from the neck mold. All of these steps and procedures are well known in the injection molding art. The parison so produced is relatively cold, having a temperature substantially below that required for successful orientation, the reason being that the said parison could otherwise not be extracted from the mold cavity nor separated from the core without significant damage to its dimensions. If the outer surface of the parison is at an elevated temperature, it tends to adhere to the mold cavity and deform in the course of extraction. If the inner surface is not cold enough, it will adhere to the core. Moreover, when extracting the core, vacuum is created in the space within the parison from which the core is extracted and therefore the parison must be strong enough not to deform under the influence of the ambient atmospheric pressure acting upon the outside thereof. In order for the parison to attain the requisite strength, it must be cooled well below that temperature at which it is desirable to form it into the finished article.
The parison so made is next subjected to a second stage of operations wherein it is heated to that temperature at which its transformation into the finished article is to take place and, once at that temperature, it is expanded under conditions imparting the desired orientation.
Such two-stage operations are well known in the art. For example, in a machine designated as RHB 5 built by the Cincinnati-Milacron Company, conventional parisons that have been previously injection molded in a conventional manner are passed through parallel banks of infrared heaters, usually while being rotated around their axes to insure improved heating and, upon reaching the desired temperature, placed into a blow mold in which a stretch mandrel extends the length of the parison to a predetermined degree, followed by expanding the extended parison into conformance with the blow mold by means of a pressure fluid. Other similarly acting devices are well known and described, e.g., in the February and March, 1976 issues of Modern Plastics (a McGraw-Hill Publication). One such device is known as the Model 650 Machine built by Nissei Plastics Industrial Co., Ltd. of Japan, a schematic description of which is given in U.S. Pat. No. 3,944,643 and U.S. Pat. No. 4,105,391. In that machine, the parison is injection molded and cooled in the injection mold which includes a core, to a temperature at which it is easily removed from said core (according to claim 1 of the above U.S. Pat. No. 3,944,643). The parison is then transferred by means of a neck mold to a heating environment designated as a reheating mold (claim 1) or heating device (Column 1, line 6), in which its temperature is raised to that desired for orientation, by means of external and, if found necessary, also internal heaters which may be of the radiant or contacting type. Once the parison reaches the desired temperature, it is transferred to a blow mold in which it is converted into the finished article by stretching and blowing, as above described in connection with the RHB 5 Machine.
The disadvantages of such two-stage operations include a slow cycle due to the fact that the parison must first be cooled from the temperature at which it is molded to near room temperature, only to be heated once more to the relatively high orientation temperature, with a corresponding waste of heat. In addition, heating of the parison is unsatisfactory because, it being made of an organic polymer and hence a poor heat conductor, uniform heating across its wall thickness is very difficult and time consuming to obtain.
In addition to the above two-stage procedures, a single stage operation may be employed, wherein the parison is cooled after having been molded only enough to lower its average temperature substantially to that chosen for orientation and the transformation of the parison into the finished article is carried out, for example, by stretching and blowing, after it has attained a uniform cross-sectional temperature distribution corresponding to the above average temperature, preferably in a tempering mold. Such single stage operation is described in my previous U.S. Pat. No. 3,966,378 according to which the parison is injection or compression molded, cooled to a predetermined, limited degree in the injection or compression mold, transferred by means of the injection core into a tempering mold for equalization of its temperature distribution and then transferred onto a stretch mandrel and into a blow mold for finishing of the final article. While such single stage operation lacks the disadvantages of excessive operating cycle, waste of heat and non-uniform temperature distribution at the time of orientation, it can benefit from improvement at the transfer of the parison into the tempering environment or mold.